(Original post by fawnbx)
Can anyone help me with what formula I use to anwser this question? ... confused! Thanks guys

No need to use any formula, just investigate it and use some logic.

The parabola has a line of symmetry (can you figure out what it is?) and since R,S have the same y coordinate, then that means they are both reflections of each other in this line of symmetry. So, since you know S, you can figure out what R must be reflecting the x-coordinate of S in the line of symmetry.